A class-D amplifier, sometimes known as a switching amplifier, is an electronic amplifier in which all transistors operate as binary switches. They are either fully on or fully off. CLASS-D amplifiers employ rail-to-rail output switching, where, ideally, their output transistors virtually always carry either zero current or zero voltage. Thus, their power dissipation is minimal, and they provide high efficiency over a wide range of power levels. Their advantageous high efficiency has propelled their use in various audio applications, from cell phones to flat screen televisions and home theater receivers. Class-D audio power amplifiers are more efficient than class-AB audio power amplifiers. Because of their greater efficiency, class-D amplifiers require smaller power supplies and eliminate heat sinks, significantly reducing overall system costs, size, and weight.
Class D audio power amplifiers convert audio signals into high-frequency pulses that switch the output in accordance with the audio input signal. Some class D amplifier use pulse width modulators (PWM) to generate a series of conditioning pulses that vary in width with the audio signal's amplitude. The varying-width pulses switch the power-output transistors at a fixed frequency. Other class D amplifiers may rely upon other types of pulse modulators. The following discussion will mainly refer to pulse width modulators, but those skilled in the art will recognize that class D amplifiers may be configured with other types of modulators.
FIG. 1 shows a simplified schematic diagram illustrating a conventional class-D amplifier 100. The input audio signals INP and INM are input to comparators 101 and 102, where input signals INP and INM are compared with triangular waves VREF generated from an oscillator 103 to generate PWM signals 106 and 107. PWM signals 106 and 107 are coupled to the gates of transistors M1, M2, M3, and M4, respectively. Output signals OUTM and OUTP of the class D amplifier are respectively provided at terminals also labeled OUTM and OUTP. As shown in FIG. 1, output signals OUTM and OUTP are connected to a speaker load 110, which is represented by an inductor L1 and a resistor R1.
The traditional class D amplifiers have outputs OUTP and OUTM, wherein each output is complementary and has a swing range from ground Vss to Vdd. The disadvantage of class-D amplification lies in the high frequency switching noise that is produced by the switching. This high frequency noise often resulted in EMI (Electronic-Magnetic Interference).
A method for reducing EMI in class-D amplifiers is described in a paper entitled, “A 20 W/Channel Class-D Amplifier With Near-Zero Common-Mode Radiated Emissions” by P. Siniscalchi and R. Hester, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 44, NO. 12, DECEMBER 2009, pp. 3264-3271. The content of this paper is incorporated by reference herein. A method for reducing EMI in class-D amplifiers is described in U.S. Pat. No. 7,355,473 to Wu, entitled, “Filterless class D power amplifier”, the content of which is incorporated by reference herein.
FIG. 2 is a waveform diagram illustrating the modulation of signals in the class-D amplifier of FIG. 1. As shown in FIG. 2, input signals, e.g., audio signals INM and INP, are compared with a triangular reference waveform VREF by two comparators as described above in connection with FIG. 1. The output signals of the comparators are pulse signals at a fixed frequency whose pulse width is proportional to the input signal. Two PWM signals are shown in FIG. 2 as OUTP and OUTM.
Filter-less Class-D Audio amplifiers, such as amplifier 100 of FIG. 1, employ a modulation scheme often referred to as BD-modulation. In this modulation scheme the output drivers, connected in bridge tied load configuration, switch the positive and negative side of the load to: 1) Vdd & GND; 2) GND & Vdd; 3) Vdd & Vdd; 4) GND & GND, where Vdd is the supply voltage and GND is the supply ground. As a result, the differential voltage across the load has three levels: 1) Vdd; 2) −Vdd; 3) 0. For 0 level audio output voltages the differential voltage across the load will be predominantly zero, allowing filter-less operation through an inductive speaker load.
However, even though the differential voltage across the load remains close to zero, the two terminals are switching simultaneously between Vdd & GND. This causes a large common mode output swing on the output terminals, shown as VCM in FIG. 2. When long wires or PCB traces are attached to the Class-D driver outputs, they will act as antennas, transmitting the fundamental and harmonic frequencies of the switching amplifier. As a result, the common-mode signal may lead to electromagnetic interference (EMI), and negatively impact the EMI criteria as specified by the FCC and European Standards.
BD modulating class-D amplifiers are sometimes referred to as “filter-free” because no LC filter is required to improve small signal efficiency. However, such filters are frequently required to reduce electromagnetic interference (EMI) in the range to comply with the FCC regulations on unintended transmitters. In audio applications, speaker wire and printed circuit board (PCB) traces become inadvertent antennas. Using output filters is a straightforward means of controlling these emissions, but also the most expensive in terms of actual cost and board area.
A conventional method for reducing EMI in class-D audio amplifiers is described by the paper by Siniscalchi and Hester, referenced above, in which the output common mode is maintained constant by using two new switches in parallel with the load. Another conventional method for reducing EMI in class-D audio amplifiers is described in U.S. Pat. No. 7,355,473 to Wu, in which a double reference wave modulation scheme is used for reducing EMI. Both methods offer their respective advantages, but require additional components and increased complexity.
From the above, it is clear that an improved method for reducing EMI in class-D amplifiers is highly desirable.